Generally, a chip resistor is mainly constituted by a cuboid-shaped insulating substrate, a pair of front electrodes, a pair of back electrodes, end surface electrodes, plating layers, a resistive element, a protective layer, etc. The pair of front electrodes are disposed on a front surface of the insulating substrate and face each other with a predetermined interval therebetween. The pair of back electrodes are disposed on aback surface of the insulating substrate and face each other with a predetermined interval therebetween. The end surface electrodes establish electrical continuity between the front electrodes and the back electrodes respectively. The plating layers cover the electrodes. The resistive element bridges the front electrodes paired with each other. The protective layer covers the resistive element. The protective layer has a two-layer structure consisting of a first insulating layer called an undercoat layer and a second insulating layer called an overcoat layer.
In chip resistors configured thus, laser light is applied to resistive elements to form trimming grooves therein. In this manner, initial resistance values which have been varied among the resistive elements in a production stage are adjusted to a target desired resistance value. In order to prevent the vicinity of the trimming groove of each of the resistive elements from being damaged by heat of the laser light on this occasion, the resistive element is covered with a first protective layer made of a glass material so that the resistive element can be irradiated with the laser light through the first protective layer. In addition, the resistive element in which the trimming groove has been formed is protected from an external environment by a second protective layer. When the second protective layer is formed of a glass material excellent in humidity resistance, the glass is required to be sintered at a high temperature of about 600° C. Therefore, there is a disadvantage that the adjusted resistance value changes to prevent the resistive element from being produced with high accuracy. To solve the disadvantage, a method for sintering a resin material such as an epoxy resin at a relatively low temperature of about 200° C. to form the second protective layer has become the mainstream recently. A contrivance has been also made as follows. That is, an epoxy resin, a polyimide resin, or the like, excellent in humidity resistance is used as the resin material to form the second protective layer which is so dense not to contain any void or any air hole.
In addition, this type of chip resistor has a configuration in which an Ag-based metal material is normally used as front electrodes and plating layers are formed to cover the front electrodes. However, a sulfide gas etc. strong in corrosiveness infiltrates gaps as boundary portions between the plating layers and a second protective layer easily. For this reason, there is a fear that the front electrodes may be corroded by the sulfide gas etc. to cause problems about a change in resistance value, disconnection, etc.
To solve the problems, the following chip resistor has been heretofore proposed, as disclosed in Patent Literature 1. That is, end surface electrodes are formed to extend up to end portions of a second protective layer, and plating layers formed on the end surface electrodes are brought into tight contact with the end portions of the second protective layer. Thus, gaps between the second protective layer and the end surface electrodes can be eliminated so as to improve corrosion resistance (particularly sulfurization resistance).